Repair docs publication bug

[edeckers]

Describe the bug

When releasing a new version of the library, doc generation for readthedocs.org fails with a dependency error.

To Reproduce

Steps to reproduce the behavior:

1. Release new version of Pyella
2. Logon to readthedocs.com
3. Open latest build
4. See error

Expected behavior

The documentation should be build and published automatically to readthedocs.org when creating a new release, without build errors.

Environment:

Not applicable

Additional context

From the readthedocs.org build log:

(...)

Installing the current project: pyella (3.1.0)

(...)

[rtd-command-info] start-time: 2025-01-09T14:06:26.565300Z, end-time: 2025-01-09T14:06:27.021396Z, duration: 0, exit-code: 2
python -m sphinx -T -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html
Running Sphinx v8.1.3
loading translations [en]... done

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyella/envs/stable/lib/python3.10/site-packages/sphinx/registry.py", line 453, in load_extension
    mod = import_module(extname)
  File "/home/docs/.asdf/installs/python/3.10.15/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1004, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'sphinx_mdinclude'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyella/envs/stable/lib/python3.10/site-packages/sphinx/cmd/build.py", line 496, in build_main
    app = Sphinx(
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyella/envs/stable/lib/python3.10/site-packages/sphinx/application.py", line 256, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyella/envs/stable/lib/python3.10/site-packages/sphinx/application.py", line 437, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyella/envs/stable/lib/python3.10/site-packages/sphinx/registry.py", line 456, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname,
sphinx.errors.ExtensionError: Could not import extension sphinx_mdinclude (exception: No module named 'sphinx_mdinclude')

Extension error:
Could not import extension sphinx_mdinclude (exception: No module named 'sphinx_mdinclude')```